CHEMISTRY COMMUNITY

alicechien_4F

A reactant is zero order if changing the concentration of that reactant has no effect on the rate constant.

alicechien_4F

Based on a previous answer given by Chem_Mod, rate laws can include catalysts if it is an elementary reaction.

Here's the link: viewtopic.php?t=1727

alicechien_4F

Did you by chance post the wrong question? I can't seem to find the corresponding question for 7B15. But in general, the B is squared because it is a second order reaction, while the A is not because it is first order.

alicechien_4F

Will the Arrhenius equation (both forms) be on the equation sheet? I can't find it on the one posted on Lavelle's website, but I know for Test 2 they also added additional equations that weren't on the posted equation sheet.

alicechien_4F

How come catalysts don't have an effect on the rate of the overall reaction if it lowers the activation energy?

alicechien_4F

If there is no solid metal in the anode or the cathode, you would add an inert electrode to that side.

alicechien_4F

I think it is -q because it is the cooling system around the chamber, rather than the chamber itself that is absorbing the heat. This means the chamber is releasing heat.

alicechien_4F

I didn't realize that the cell diagram does not represent the order of the molecules in the reaction. But a rule I can follow is that the side being oxidized is the anode, and whatever solid that is on that side should be on the side of the anode, right? And that if there is no solid then the elect...

alicechien_4F

The anode should be on the left side of the cell diagram, and the cathode should be on the right. The Ecell should be cathode - anode and for galvanic cells it should always be positive.

alicechien_4F

What sections of the outline can we do for this week's homework? Can we do electrochem since the test is this week, or should it be all kinetics?

alicechien_4F

The single bar represent the species that has undergone a state change, while the double bar is a salt bridge. The double line is usually also used to separate the anode/oxidation reaction from the cathode/reduction equation. You use the commas when there is no phase change, and you would add a sing...

alicechien_4F

It is similar to the density of water. Even though you change how many electrons (by changing the coefficient), it won't affect the the E nought value of the each material.

alicechien_4F

Test 2 is technically not cumulative, but there are many concepts from the midterm that are also applicable to the material after the midterm. For example, we also talked about Gibbs, entropy, and enthalpy, so it would be good to brush up on those concepts.

alicechien_4F

I believe the condition was if you do not have a solid on either the anode or cathode side, you add it in. I'm not sure why they included platinum on the right side, but on the left side it is included because both molecules are aqueous.

alicechien_4F

I don't think you can anymore, but you can try emailing your TA last quarter to see if it is still possible.

alicechien_4F

It is similar to why we balance all other atoms in a chemical reaction. It is so that conservation of energy, and in this case, the charge of the electrons, is maintained.

alicechien_4F

Test 2 isn't technically cumulative, but what we are learning right now (Van't Hoff) requires knowledge of midterm material (enthalpy, Gibbs, PV = nRT), so it would be good to review that content since everything builds on each other.

alicechien_4F

I think re-reading the question and writing down the equations is a great way to start! What I do is write down all the given information in the problems (ex: pressure, temperature, whether it's isothermal, etc.) because sometimes writing the given info down allows you to put values into the equatio...

alicechien_4F

You have to convert from Kp to Kc because the values are different. I believe Kp and Kc are only the same when the number of moles of gas on the reactant and products sides are equal (correct me if I'm wrong!). Van't Hoff's equation relates Kc to change in temperature.

alicechien_4F

I know a lot of people who have taken 14BL and 14C concurrently! Most people think 14C is a lighter in terms of content than 14D, and 14BL is more tedious work with all the labs, but I think with 14B content fresh, you'll be ok!

alicechien_4F

One way is to use the Gibbs of formation for each molecule and do products - reactants. Another way to find delta G is to use the equation delta(G) = delta(H) - T*delta(S).

alicechien_4F

At equilibrium, the forward and reverse reactions occur at the same rate. This means that neither reaction is favored, and so neither reaction is spontaneous.

alicechien_4F

Is this related to the relationship between ∆G and the direction of a spontaneous reaction? Does a spontaneous reaction have a -∆G and proceed in the forward direction? Yes, a spontaneous reaction will always have a negative Gibbs value and proceed in the forward direction. If the reaction has a po...

alicechien_4F

Since entropy is a state function, the change in entropy for an irreversible process is the same as a reversible process.

alicechien_4F

If you know that the equation is at equilibrium, then delta G = 0. Also, if all of the molecules are at their most stable state (ex: O2 as a gas) the Gibbs value is also zero.

alicechien_4F

In the Wednesday Lecture, there was a side by side comparison of two graphs showing the irreversible and reversible pathway for work. The reversible pathway had constant T along the pathway, V increasing, and P decreasing. The irreversible pathway had T changing along the pathway (decrease and then ...

alicechien_4F

Reversible work is also isothermal, while irreversible work is not. Irreversible work just has constant pressure.

alicechien_4F

For problems like 4F.1 and 4F.3, why does change in entropy decrease when temperature is increased? I understand that the equation delta(S) = q/T so increasing the denominator will increase delta(S), but can someone explain conceptually why entropy decreases when temperature increases? I thought hig...

alicechien_4F

Can a reaction be isothermal and isobaric at the same time? If so, which equations would you use for work and enthalpy?

alicechien_4F

For 4.17, it asks us "Calculate the standard entropy of vaporization of water at 85 degrees C, given that its standard entropy of vaporization at 100 degrees C is 109.0 J/Kmol and the molar heat capacities at constant pressure of liquid water and water vapor are 75.3 J/Kmol and 33.6 J/Kmol, res...

alicechien_4F

Why is Cp greater than Cv for ideal gases? I understand that Cp = Cv + R, but can some explain conceptually why? Thanks!

alicechien_4F

For question 4C.3, it asks "Calculate the final temperature and the change in enthalpy when 765 J of energy is transferred as heat to 0.820 mol Kr(g) at 298 K and 1.00 atm (a) at constant pressure; (b) at constant vol- ume. Treat the gas as ideal." I understand how to calculate the final t...

alicechien_4F

The phase change from solid to liquid releases only 6.01 KJ/mol, but the change to vapor releases 46.7KJ/mol, making steam burns much more severe.

alicechien_4F

When you form a bond, you release energy. On the other hand, when you break bonds, you must add energy. This is because atoms/molecules are more stable when they are bonded together, so to break bonds you must add. When you form bonds, electrons typically go to lower, more stable orbitals, so it rel...

alicechien_4F

Leila_4E wrote:Is the table we find this in the textbook or is there a general one online that is the same?

I would go with the table in the textbook for homework problems because sometimes the bond enthalpies differ slightly online. It should be roughly the same.

alicechien_4F

Leila_4E wrote:And -q means heat out of the system and into the surroundings, right?

-Q means exothermic reaction, so the system loses heat while the surroundings gain heat, making it appear warmer.

alicechien_4F

Chantel_2I wrote:Is there a reason why the horizontal line for vaporizing water is so much longer than the one for melting ice?

It takes longer to vaporize water because there is a larger enthalpy, which means more heat has to be applied at a constant rate.

alicechien_4F

Can we use Kc for all of the equilibrium problems that have gases in the reactant/products instead of Kp? I don't remember going over converting Kc to Kp in lecture or discussion in depth, so would we have to do it on the test?

alicechien_4F

You're given the amount of reactants and products in moles, so you have to divide by 3.0 L to obtain the molarity. Hope this helps!

alicechien_4F

Can someone clarify what Dr. Lavelle said in lecture about how a solution with [H3O+] < 10^7 M is considered neutral? Does this only apply to acid reactions or how does it work if it is a basic solution with a lot amount of H3O+? Thanks!

alicechien_4F

All gas reactants/products are included in the K expression, including water vapor. The only things not included are solids, liquids, or solvents. Hope this helps!

alicechien_4F

If you are dealing with a weak acid or base, you would also use an ICE table because you don't know how much dissociates (versus a strong acid/base that dissociates 100%).

alicechien_4F

Le Chatelier's Principle applies to temperature, concentration, and pressure, but it doesn't apply to catalysts because catalysts speed up the forward and reverse reactions at the same rate.

alicechien_4F

The way I learned the "easy" way in high school was that the gas wants to go into the area with more space so that it can expand. That is why the reaction will proceed towards the side with less moles, since there is more area for the gas.

alicechien_4F

Angela Wu-2H wrote:Can partial pressure be measured in bars and atm or just bars?

Partial pressure can be measure in both bars and atm. I believe for this class, we will assume 1 bar = 1 atm, but you can also convert if necessary.

alicechien_4F

I think it is easy to mix up Q and K because they're calculated the same way! However, Q is used to calculate the ratio between products and reactants at any given time, while K is used to calculate the ratio between products and reactants exactly at equilibrium. Q will always proceed towards K in o...

alicechien_4F

A molecule is more thermodynamically stable if it has a smaller K value. The smaller the K value, the less product there is relative to reactant. That means the reactant (in this case, Cl2 and F2) will dissociate less and remain as reactants.

alicechien_4F

What is the difference between shifting/changing the equilibrium versus changing the equilibrium constant? Why does shifting the equilibrium sometimes not change the equilibrium constant?

alicechien_4F

What is an easy way to remember which side of the chemical equation heat is on? Also, why do exothermic equations have a negative value?

alicechien_4F

Why is water included in equilibrium when it is in a gaseous state but not when it is in a liquid state? Is water ever a solvent when it is in its gaseous state?

alicechien_4F

Why does the equilibrium shift to the side with less moles of gas when pressure is increased?

alicechien_4F

For the modules, if we were given an equation like A (g) + B (g) --> C (s), why do we write the equilibrium constant as 1/[A][B] instead of just [A][B]? I thought we were supposed to just take solids and liquids out of the equilibrium constant.

alicechien_4F

Is the unit "bars" a unit of concentration or do we have to convert it when solving partial pressure problems?

alicechien_4F

It depends on the pH of the solution that it is in. You have to compare the pKa with the pH to see if the acid will protonate or be deprotonated. Hope this helps!

alicechien_4F

On the final if it asks us to identify acids/bases, do we assume they are using the Lewis definition or the Arrhenius definition?

alicechien_4F

I think the best way to determine whether a compound is amphoteric is to draw out the lewis structure so that you can identify any lone pairs or any hydrogens that can be given off. Other than that, if you know a compound contains a common base (like an amine group in amino acids) and a common acid ...

alicechien_4F

When do we use a one-way arrow and when do we use an arrow pointing in both directions?

alicechien_4F

Do we need to memorize hydrocarbons for coordinate compounds? For example, do we need to know the formula for "ethylene" in ethylenediamine and how to draw it? If we do, what other common hydrocarbons should we know? Thanks!

alicechien_4F

Brosted acid and bases are defined by their ability to accept or donate hydrogen ions. Lewis acid and bases are defined by their ability to accept or donate electron pairs. A Lewis Acid is an electron-pair acceptor. A Brosted acid is a substance that can donate a hydrogen ion.

alicechien_4F

How does this term relate to ligands that are discussion in terms of biology? A ligand in chemistry is an ion/molecule that is attached to a metal through coordinate covalent bonds. Similarly, a ligand in biology is a molecule that binds to a larger molecule. Both involve smaller molecules binding ...

alicechien_4F

I think it is chloride and not chloro because it is not part of the coordination compound and is written outside the brackets.

alicechien_4F

You have to draw out the Lewis structure and see how many lone pair there are. You should use VESPR to think about the shape and see whether the multiple lone pairs can orient themselves to bind to another molecule.

alicechien_4F

I think we will need to be able to name coordination compounds and also how to write their chemical formula, which is what the homework is on. We probably will need to know whether coordinations are polydentate or not as well.

alicechien_4F

Most hydrocarbons are chains, so you would put both carbons as "central" atoms. Because you want symmetry, you would bond two H atoms to each of the carbon atoms. Then, add as many bonds as you need between the two carbons for them to fill an octect.

alicechien_4F

Yes, the angles would be less than 120 and less than 90 because the lone pairs causes a slight electron repulsion.

alicechien_4F

Is ICl₂^- polar or non-polar? I know that it is symmetrical with Cls on both side of the I atom, but do the 3 lone pairs create a dipole moment because it's an odd number? Thanks!

alicechien_4F

I believe we only need to know up to octahedral. Dr. Lavelle made a post with the list of molecular geometries to know for the test. Hope this helps!

alicechien_4F

The Cl atom has a much higher electronegativity than H and has more electrons in its valence shell (8 vs. 2) after bonding, so there will be a stronger negative charge on that end of the atom. Hope this helps!

alicechien_4F

The two words are interchangeable and refer to a molecule that has lone pairs that distort the bond angles. Hope this helps!

alicechien_4F

All IMFs are between either nonpolar or polar molecules. Within that, there are hydrogen bonds, dipole-dipole, London Dispersion Forces, and ion-dipole. Ion-dipole is the interaction between ion and partial charges on a polar molecule. Dipole-dipole interaction are interactions between partial charg...

alicechien_4F

Why are double and tripe bonding pairs still considered a single region of electron density and not affect the shape/bond angles? I'm confused because I thought if you had more electrons there would be a greater repulsion, so it would affect the shape and the bond angles. Thank you!

alicechien_4F

For part D of 3F.1, the answer for SO₂ includes dipole-dipole forces. I thought there were only LDF forces because it is symmetric. Is there a way to know if there are dipole-dipole interactions without knowing the shape of the molecule?

alicechien_4F

Hydrogen bonding is so common because it stabilizes important biological systems. The atoms involved in hydrogen bonding (H, N, O) are very common in biological structures such as DNA, RNA, and proteins which is why we see it so often.

alicechien_4F

Electrons repel each other, so delocalizing them over more bonds decreases the electron-electron repulsion in any one bond. Because the electrons can move around, they can satisfy the octet for more atoms and make them more stable.

alicechien_4F

Electron affinity is the amount of energy that is released when an electron is added to an atom. It is also thought of as the likelihood of an atom gaining an electron. Electronegativity is how strongly an atom pulls on its bonding electrons instead of gaining electrons.

alicechien_4F

Effective nuclear charge increases as you go across a period because the amount of protons in the nucleus increases, which the pull on the electrons. Effective nuclear charge decreases down a group because the shielding effect of newly added shells means that electrons on the valence shell are not c...

alicechien_4F

So the photons are acting as particles when emitted in the photoelectric effect, but I also wrote in my notes that electrons act as waves. Does it acts as both a wave and a particle or did I misunderstand the lecture? We typically treat electrons as particles, but because all objects have wavelengt...

alicechien_4F

Lavelle will provide all the information that we need for specific problems. My TA said that he will provide electronegativity values unless it is very obvious based on the periodic trends. I don't think you have to memorize the specific bond lengths, but understand them conceptually.

alicechien_4F

For the lewis structures we've done for practice, we've only done resonance structures for compounds that have one element surrounded by multiple identical atoms of another element (like CO 3 - or O 3 ). Do we also draw resonance structures for compounds that are more complex or not symmetrical (lik...

alicechien_4F

The noble gases do not follow the electron affinity trend because they have a full set of valence electrons. They are also often excluded from electronegativity trends because they do not bond since their shell is full. Hope this helps!

alicechien_4F

We have only used the work function in the context of the photoelectric effect, so if you see it you can assume you will be using the Energy/KE equation. Hope this helps!

alicechien_4F

Yes, elements in the same group will have the same exception because their electron configurations are similar in the amount of electrons and therefore have similar properties. Hope this helps!

alicechien_4F

You multiply by Avogadro's number since you're looking for the energy of a single electron, not a mol. That way, the units will also cancel out. Hope this helps!

alicechien_4F

The way I remember it is that atomic radius increases from top right to bottom left, and all the other trends increase the opposite way, so from bottom left to top right!

alicechien_4F

so at each energy level, are there s- p- d- f- orbitals? In the n=1 energy level, there is only the s-orbital. In n=2, there is only the s-orbital and p-orbital. In n=3, there is only the s-, p-, d-orbitals. Another way to tell which orbitals are present at an energy level is by using the quantum n...

alicechien_4F

Along with Cr and Cu, all of the elements that are in the same group underneath those elements exhibit the same exception for their electron configuration.

alicechien_4F

Since ionization energy is the energy required to remove an electron from a neutral atom, atoms of the left side of the period table would have the lowest ionization energy because alkali metals want to lose electrons to maintain a full valence shell and become a cation. The lower the ionization ene...

alicechien_4F

What is the relationship between electronegativity and ionization energy? I know Dr. Lavelle said that the atom with the lowest ionization energy is the central atom for a lewis structure, but in high school I learned that the central atom was the most electronegative atom. Is the atom with the lowe...

alicechien_4F

Ionic radii is the radius between two ions that are ionically bonded (ex: NaCl). Atomic radius is the radius between two neutral atoms. Hope this helps!

alicechien_4F

So avogadro's number is how you get it in the correct mole qty? In part A, you're solving for the amount of energy emitted by one sodium atom. In part B, you're solving for the amount of energy emitted by 5.00 mg of sodium, so you have to convert 5.00 mg to moles, and then multiply by avogadro's nu...

alicechien_4F

I think in high school many teachers would teach us to write 4s before 3d because that is the order that the shells were filled (and seemed more logical without any experimental data), rather than by the amount of energy. For Zinc, it would still be [Ar] 3d 10 4s 2 because for elements that have an ...

alicechien_4F

The n represents the principle quantum number that determines energy and size. It is also called the shell, and it corresponds with the row number for an atom on the period table as well as the coefficent of the subshell notation. The l represents the angular momentum quantum number, where l = 0 is ...

alicechien_4F

The angular momentum quantum number (l) corresponds with the subshells so that l = 0 is the s-orbital, l = 1 is the p-orbital, l = 2 is the d-orbital, and l = 3 is the f-orbital. We know that the s-subshell has 1 orbital, the p-subshell has 3 orbitals, the d-subshell has 5 orbitals, and the f-subshe...

alicechien_4F

I don't understand what to use given in the problem to get the answer. I thought b was 11 because if n=6 and l=5, then shouldn't ml = -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5? Also can someone explain c and d too? L is the angular momentum quantum #, and although the allowed values go from l = 0, 1, 2,...

alicechien_4F

Just to clarify, if there is excess energy after the electron is ejected, that energy would be converted to kinetic energy (Ek). The v in the equation Ek=1/2(mv^2) refers to the velocity of the electron, correct? Because surely it is not referring to nu, as in E=hv. Correct, the v refers to the vel...

alicechien_4F

Hi Doris! I think the easiest way to match the wave to the activity is by using Hertz since most EM spectrum charts label the radiation based on hertz. Also, the answer key actually matches 3.3E-19 J (which has a frequency of 5.0E14 Hz) to reading instead of microwave, which makes sense because ligh...

alicechien_4F

Can somebody explain to me why it isn't B? The answer (b) states that the wavelength of the radiation decreases. This not correct because with the equation C = λv, wavelength (λ) and frequency (v) are inversely proportional. If you decrease the frequency, the λ must increase to compensate since C (...

alicechien_4F

In Lavelle's lecture, I was confused on what the work function is. Can someone help clarify what it is and how to solve for it?

alicechien_4F

All EM waves have radiation, but only wavelengths that are shorter than visible light are harmful to humans. Although many people believe microwaves can cause cancer, it is unfounded because the wavelength of microwaves is significantly longer than visible light and the energy of individual photons ...

alicechien_4F

Even though the ratios are the same, it's important to differentiate between empirical and molecular formula. Like what the people above said, empirical formula is used to find the ratio of atoms. When you're given mass percentages of a unknown compound, you'd find the empirical formula first before...

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